There is a need for more powerful sources of extreme ultraviolet (EUV) light at 13.5 nm in order to increase the throughput of semiconductor patterning via the process of EUV Lithography. Many different source designs have been proposed and tested (see historical summary for background [1]) including the highly efficient (up to 30%) direct discharge (DPP) lithium approach [2, 3, 4, 5, 6, 7] and also laser-plasma (LPP) irradiation of tin-containing [8] or pure tin droplets [9, 10, 11]. Laser irradiation of tin droplets has been the subject of intensive recent development [12, 13], particularly in the pre-pulse variant [11], which has a demonstrated efficiency of 4% and a theoretical efficiency of up to 6%.
In both lithium DPP and tin LPP approaches it is necessary to keep metal atoms from condensing on the collection mirror that faces the EUV-emitting plasma. Also, in the tin LPP approach, but not with lithium DPP, there are fast ions ranging up to 5 keV that have to be stopped otherwise the collection mirror suffers sputter erosion. The design of a successful EUV source based on a metal vapor must strictly protect against deposition on the collector of even 1 nm of metal in days and weeks of operation, and this factor provides the most critical constraint on all of the physics that can occur in a high power source.
Many magnetic field configurations have been discussed [14-29], with and without a buffer gas, to trap and exhaust tin ions. Methods have been proposed [14, 30, 31] to further ionize tin atoms so that they may be controlled by an applied magnetic field.
The symmetrical magnetic mirror trap [15, 18] has a limited cross sectional area for plasma exhaust toward each end, implying a very high concentration of plasma heat at each end where particle traps have to condense the working substance of the LPP source, usually tin. The condensation surfaces may become coated with tin during operation, and there can be sputtering of tin atoms associated with the impact of plasma tin ions that are accelerated toward the condensation surface by a plasma sheath potential. In one typical example, with a low hydrogen pressure to moderate the sheath potential [34] there can be Sn3+ ions falling through a 12 volt sheath potential to deliver a sputter energy of 36 eV. It is possible that some of these sputtered tin atoms are able to cross the magnetic field to reach the adjacent part of the collection mirror, reducing collection efficiency, an effect reported by Mizoguchi et al. [15].